linux-kernel - Re: [PATCH RFC 0/7] rcu: v2 Inlinable preemptible rcu_read_lock() and rcu_read

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Message-ID: <20120414172504.GH2452@linux.vnet.ibm.com>
Date:	Sat, 14 Apr 2012 10:25:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [PATCH RFC 0/7] rcu: v2 Inlinable preemptible rcu_read_lock()
 and rcu_read_unlock()

On Sat, Apr 14, 2012 at 10:08:54AM -0700, Linus Torvalds wrote:
> On Sat, Apr 14, 2012 at 9:58 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I'll see if it boots and what it does to my profiles and
> > microbenchmark, though.
> 
> Ok, I can't see any performance difference in the numbers - my
> benchmark noise is *much* bigger than anything this would show.

Might still be worthwhile on embedded CPUs that don't optimize
function calls as thoroughly as does x86, maybe?

> The profile looks fine, and obviously __rcu_read_lock() is entirely
> gone. The top user (avc_has_perm_flags()) looks fine. I note that you
> might want to look at the placement of the percpu data - I think it
> probably makes sense to put the RCU data close to 'current' etc to get
> as much cacheline sharing as possible, and it doesn't seem to be right
> now, but it looks reasonable.

Is there somewhere in non-architecture-specific code that would be a
good place to put this?  Or is the DEFINE_PER_CPU() for current_task
moving from arch/x86 to core code?

> But on the whole, I can't claim that it looks noticeable ;*(

Well, then, I guess I don't feel quite so bad about having prioritized
this so low for so long.  ;-)

							Thanx, Paul

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